The Númenóreans used Sindarin like a status symbol. The wealthy and nobles spoke it to each other and used it as a formal language of law, along side of Quenya, which was even "higher" to them. Therefore, anyone one with an education or some money would name themselves and their children in Sindarin to showcase their status. However, they had a bit of a problem. There are two sounds that appear in Sindarin that don't in their native tongue; and they had difficulty speaking them: CH and Y. That is why we see words like "Rohan" and "Rohir" instead of "Rochan" and "Rochir". For the Númenóreans' and their descendants' names, simply change the CH's to H's and the Y's to I's.